This invention relates generally to a closure that diverts and prevents surface runoff or other liquids from entering a sewer system through the manhole opening, while permitting pressure equalization at all times between the sewer system pressure and atmospheric pressure. The advantages of such a closure are fully described in applicant's previously issued U.S. Pat. No. 4,030,851 issued June 21, 1977.
Heretofore in order to achieve and ensure bi-directional gas pressure equalization, i.e. relieving excess pressure in the sewer system to the atmosphere or supplying air from the atmosphere if sewer system pressure drops below atmospheric pressure, closures with either two individual (oppositely directional) pneumatic valves or a single valve with multiple moveable elements were employed. This practice increases manufacturing costs and reduces reliability.
The present invention overcomes problems of the prior art by providing a closure having one pneumatic valve with a single moveable valve element that can be efficiently manufactured and installed at lower cost while increasing system reliability.